Numerous devices are already available for controlling the discharge and flow of molten metals from a vessel.
In the case of a very early system for casting steel or the like, use is made of a stopper mechanism in which the outlet aperture in the bottom of the vessel is adapted to be closed off by a stopper located in the interior of the vessel, the stopper being secured to the lower end of a rod. By means of a system of levers adapted to be actuated from the outside, the stopper may be raised for pouring and may be lowered again to close the outlet. The disadvantage of this system is that flow control and the shutoff safety is unsatisfactory, for example as a result of the formation of deposits or wear upon the stopper.
It has also already been proposed to use rotating valves by means of which an eccentric inlet duct can be brought into communication with an outlet aperture by a rotating connection. This requires very accurate machining and grinding of a difficult spherical joint between the rotating and the stationary components. Furthermore, the molten metal tends to solidify in the inlet aperture.
Also known are sliding closures built onto the bottom of the vessel containing the molten metal, but the closure elements, which slide one upon the other under preload are subject to considerable wear since movement of the adjustable parts must take place at the high temperatures of the molten metal. Another disadvantage is the high procurement and maintenance costs. Great accuracy in the machining of the slides, which are made of refractory material, is also required in order to achieve reliable sealing.
Another problem arising during the casting of molten metals is the need to prevent slag, and other non-metallic inclusions, from being carried along. Many attempts have been made to solve this problem. For example, it is known to use tundishes with partition-like displacement elements in order to promote separation of non-metallic inclusions in the molten metal. It has been found, in practice, that the carrying along of non-metallic inclusions by suction in the discharge area cannot be prevented. Apart from this, building up the dams and weirs after each casting cycle is very costly and time-consuming.
It has also been proposed to keep the slag away from the outlet by injecting an inert gas, but this involves a relatively major technical effort and the results are questionable. It is also known to arrange, concentrically with the discharge duct, a sensor based upon electromagnetism. This makes it possible to evaluate the difference in measurements of molten metal and slag, so that, when slag is detected, the casting process is halted. It is particularly difficult to introduce such sensors in areas of wear in the outlet duct. Furthermore, a certain amount of slag has to pass through the duct before it can be detected.
There is also the requirement that the molten metal should preferably not come into contact with air.
Another problem is that in the case of tundishes comprising one inlet and several outlets, the temperature of the molten metal at different outlets varies and this is undesirable.
Even if there is only one outlet, some of the molten metal flows directly from the inlet to the outlet and will therefore be at a temperature higher than that of metal circulating for some time in dead areas.
Separating non-metallic inclusions may also raise problems if the period of residence in the metallurgical vessel is too short, or if the melt is highly turbulent, since such inclusions require a certain amount of time to rise to the surface of the melt.
The components used to form the plugs for the outlets and their support elements can be of substantial size and hence heavy; for some installations, it is necessary to use hoists to place them in position It is frequently difficult to readjust the movable components after they are installed, and when a metal melt is filled in the vessel. Due to the high operating temperatures, however, the valve rod may bend or otherwise deform.
The tundishes are made of refractory material. When they are fitted in position, for example with refractory mortar or the like, positional tolerances may arise. Further, the entire metallurgical vessel may deform or warp, in the severe operating conditions encountered in foundry operation, and particularly in furnace operations for steel. Positional deviations, unintended and unavoidable, may arise which have to be compensated so that operating elements and especially elements subject to breakage and which are somewhat brittle do not break under bending stresses.